Add simple mechanism for synthetic recents transitions

- Only the simplest flow to unblock development, this synthetic transition
  path is immediately interrupted when any real transition is started
- Also remove a bunch of unused code

Bug: 366021931
Flag: EXEMPT (unused without enableFallbackOverviewInWindow)
Tests: atest RecentsTransitionHandlerTests
Change-Id: I4444fef508e6477685c71d4798318273039b1af8